Lightsaber effect in cycles without compositing?
I'd say postprocessing, simple glow... but if you really wan't pure cycles:

1) Start with a cube and 3d spherical gradient texture with coordinates set to Object (Absorption shader with inverted colors are here for simple visualisation):


this gives us a smoothed sphere inside the cube:


2) Scale texture coordinates in Z by some really low value - this will stretch the sphere texture in that axis, giving us something similar to cylinder:


3) remove Invert Color and Volume Absorption shader nodes (which were used just to see what is really going on with the texture when scaling and using Object Texture coordinates) and plugin Mix shader node with Transparent and Emission shaders. Plug the output to Volume socket:


4) It still is centered stretched sphere, but the transparency interpolation is too linear - lets try different interpolation (by powering linear values):


5) Interpolation is much better, but the overall emission strength is way too low now - lets multiply the values now, and also stretch the cube in Z axis to desired shape:

Tweaking math nodes Power vs Multiply should give you expected smoothing and overall emission power.

6) Simply change emission color and lightsaber is ready to use: